Constant frequency generator



- March 18, 1941. 11 553 2,235,317

:' ICONSTANT FREQUENCY GENERATOR Original Filed June 17, 1937 Rig. 1.

v 2 Sheets-Sheet l INVENTOR.

Thomas B Gibbs a: 3?

1 /4Z3 A5? A95 #10 A92 Wi y q hu/ March 18, 1941. 'r. a. GIBBS CQNSTANT FREQUENCY GENERATOR Original Filed June 17, 1937 2 She'ets-Sheet 2 INVENTOR. Thomas B. Gi Y wag J:

Patented Mar. 18, 1941 UNITED STATES PATENT OFFICE Original application June 17, 1937, Serial No. 148,747. Divided and this application March 31, 1939, Serial No. 265,361

9 Claims.

The present invention relates in general to constant frequency generators, and more in particular to a generator of this character which F has an output within the commercial range of frequencies, such as 60 cycles per second, which is suitable for running a small motor.

An object of the invention is the production of a constant frequency generator which is suitable for running the motor which drives the rotatable parts of a watch timing apparatus,

Another object of the invention is the production of a constant frequency generatorof the foregoing type, in which the primary current r source which ,det'ermines the output frequency is actuated by a master watch.

A special object of the invention is to provide means whereby a master watch may be used to operate a synchronous motor at a constant speed.

The invention is shown and described herein as part of a watch timing apparatus, for which it was especially designed, but it may be used for other purposes. Only so much of the watch timing apparatus is shown herein as is necessary for an understanding of the invention. Further details of the watch timing apparatus will be found in my co-pending application, Ser. No.

148,747, filed June 17, 1937, of which the instant application is a division.

The invention will be described hereinafter with reference to the accompanying drawings, in which- Fig. 1 is a top view of the special tuning fork used in the constant frequency generator;

Fig. 2 is a side view of the tuning fork;

Fig. 3 is an end view of the tuning fork, taken from the right in Fig. 2;

Fig. 4 is a section through the crystal microphone unit associated with the tuning fork, taken 40 on the line 44, Fig, 5;

Fig. 5 is another section through the crystal unit, taken on the line 5-5, Fig. 4; and

Fig, 6 is a circuit drawing showing in diagrammatic form the complete generator and watch timing apparatus.

Reference will first be made to Figs. 1 to 5, inclusive, for the purpose of describing the tuning fork which forms a component part of the constant frequency generator. The complete tuning fork, so-called, includes the fork proper, a driving magnet, a crystal pickup unit, a terminal strip, and a suitable base on which the parts are mounted.

The fork comprises the tines I04 and I05 and the heel I06, and may be cast in the form shown, preferably of a special ferrous alloy having such characteristics that the frequency of the fork is substantially independent of temperature changes over a wide range. The tines I04 and I05 are provided with weights I01 and I08,

respectively, for convenience in tuning the fork at the factory. These weights need not be adjusted afterwards, except under unusual circumstances.

The base I00 for supporting the fork and other parts is preferably a casting in the form shown, and has a pedestal I02 at one end on which the fork is mounted. The top of the pedestal has a shallow groove or recess to receive the heel I06 of the fork, which is held securely in'place by means of two screws I03.

The drive magnet is indicated by the reference character H3 and is preferably of laminated construction. The magnet has a winding II I and two pole pieces H4 and H5. The pole pieces are secured to the core by means of small screws which also serve to hold the laminations together at the ends. The magnet is supported by means of two small posts I09 and H0 which are part of the base, and is rigidly secured to these posts by means of bolts I II and H2.

The pickup device is a crystal microphone unit I2I, which is similar to the crystal unit disclosed in my prior application, Ser. No. 124,570, filed February 8, 1937. The so-called crystal which constitutes the operative element of the microphone unit comprises two square sheets or plates I36 and I31, Fig. 5, of Rochelle salt, cemented together with a layer of tin foil I38 between them. There is also a layer of foil I34 on one side and a layer of foil I35 on the other. There is a thin strip of brass I50 assembled next to the central foil I30. and underneath the foils I34 and I35 are assembled thin brass strips I39 and I40, respectively. The brass strips I30 and I40 are directly opposite one another, and their ends are brought together and are soldered to the terminal strip I H. The brass strip I50 is soldered to a terminal strip I5I. At one corner of the assembled crystal there is cemented a member I42, to which is attached a short pin or rod I43.

The crystal is contained in a sealed receptacle comprising the two metal plates I30 and I33 and the rubber rings I3I and I32. The construction of the receptacle is clearly shown in Figs. 4 and 5. The crystal is supported by six rubber blocks such as I52, three above and three below. The terminal strip I5I passes out between the rubber rings I3I and I32, as seen in Fig. 4. The terminal strip I is bent, as shown in Fig. 5, and is assembled between the ring I32 and the metal plate I33. It is cut off short enough so that it does not pass outside the receptacle, The pin I43 passes out through an opening in plate I33, this opening being provided with a soft rubber bushing surrounding the pin I43. The receptacle with the crystal contained therein is subjected to compression after assembly, and the plates I30 and I "are then secured together by means of four tubular rivets I44I41. Thus the terminal strips are clamped in position and the crystal is resiliently supported at threee corners by the rubber blocks I52, which are compressed between the plates I30 and I33.

The complete crystal unit I2I is mounted ad iacent the tuning fork on a support I20, to

which it is secured by means of four screws such as I43, as seen in Figs. 1 and 2. The support I20 may be cast as part of the base I00. The pin I43 is cut oil to the proper length so that when the unit I2I is in position the pin will lust reach the tine I05 of the fork. -A spot on this tine has previously been tinned, so that the end of the pin is readily secured to the tine by a drop of solder.,

A terminal strip is indicated by reference character ISI, and comprises a flat strip of'flber I6I and four terminal lugs I82-I65. The. strip is secured to thebase I by means of two screws I66 and I61. Two tubular spacers I08 and I69 may be inserted between the strip and the base. The terminal II of the crystal unit may be connected to terminal lug I62. The other terminal I is grounded on. the frame, and the frame is therefore connected to lug I63 by a short conductor secured under the head of screw I61, unless the base is otherwise grounded. The terminals I62 and I63 therefore are the output terminals for the crystal unit. The winding III may be connected to terminals I65 and "$4.. The tuning fork is driven by impulses generated responsive to the ticking of a master watch, as will be explained presently, but for various reasons the frequency of the fork must be considerably higher than the beat frequency of the watch, which is five cycles per second. For one thing, it is extremely difficult, if not impossible, to amplify a five-cycle alternating current and produce an output current suitable for operating a synchronous motor. The equipment required is very expensive, and the results obtained I even with the best apparatus available are unsatisfactory. Another consideration that renders a higher frequency essential is the hunting that is always present in a synchronous motor. With a very low frequency, on the order of five .cycles per second, the number of poles on the motor is so low that the hunting makes a watch timing apparatus inoperative for all practical purposes. The frequency must not be so high, however, as to make it impracticable to drive the fork from the beats of the master watch, and it must of course be an exact multiple of the beat frequency. Experiments have shown that the foregoing conditions are met satisfactorily if the fork is tuned to a frequency of 60 cycles per second. The fork is So designed that its natural frequency is very close to 60 cycles, and it is then tuned exactly by adjusting the weights I01 and I08. 3

Referring now to Fig. 6, the timer and tuning fork are shown diagrammatically within the dotted enclosures labeled T and F, respectively. The timer is a unit by itself and is shown in my parent application previously referred to. The tuning fork is also a complete unit, but is preferably mounted along with other items in a large casing which may be referred to as a power cabinet. These other items include the amplifier Al, the voltage regulator VR, the power amplifier PA, the rectifier Ryfihd the amplifier A2. As the power cabinet may be of any suitable design and construction, such as is commonly used to house amplifiers, radio sets, etc., it has not been shown.

condenser 208.

The microphone 200, used for picking up the beats from the master watch, may be mounted in the power cabinet in a shockproof compartment, as shown in my eopending application, Ser. No'. 124,571, filed February 8, 1937. This microphone, as well as'the test watch microphone 280, is preferably of the type shown in my copending application, Ser. No. 124,571, filed February 8, 1937. This latter application shows the microphone complete with stand and cord for connecting it up to an amplifier. If desired, the stand may be enlarged so as to include the compartment for the master watch microphone;

and if this is done, a single two-conductor cord will sufllce for connecting both microphones to the power-cabinet. This cord, it will be understood, should be provided with a metallic sheathing in order to shield the conductors and ground the-casings of the microphones.

- The amplifier AI comprises a Type 57 pentode 202, a Type 608G double triode 203, and a Type 885 thyratron 204. The tubes mentioned have been used in practice and operate very satisfactorily, although it will be understood that other types of tubes which are on the market could also be used.

The circuit connections for the pentode 202 are similar to those shown in my prior application hereinbefore referred to, but may be described briefly nevertheless. One side of the crystal unit in the microphone 200 is connected to the casing and thence to ground, as shown, and the other side is connected over a conductor 20I to the grid of the pentode. The conductor 20I should be shielded as previously mentioned. The plate of the tube is connected to the plus B lead through a resistor 2I0. The suppressor grid, the grid next to the plate, is connected to the cathode, and the latter is connected to ground through a resistor 207, which is an arrangement commonly employed to provide a negative grid bias. The resistor 20! is shunted by a by-pass The screen grid of the tube is connected to the plus B lead at a point between the two bleeder resistances 210 and 2", so that less thanthe voltage at the plate. The control grid is connected to ground. through a high resistance grid leak 206.

The plate circuit of the pentode 202 is coupled to the grid of the left triode of tube 203 by means of a condenser 2| I. The tube 203 constitutes in effect two ordinary triodes with conventional circuit connections which need not be described in detail. The plate of the left-hand triode is coupled to the grid of the right-hand triode through a condenser 2I3 and the potentiometer .2I2. The latter functions as a grid leak for the grid of the right-hand triode and also as a volume control.

The plate circuit of the right-hand triode of the tube 203 is coupled to the control grid of the thyratron 204 by means of the condenser 2I5. This control grid is provided with a negative potential over a conductor 25! and through the resistance 2I6, sufllciently to normally bias it to cut off condition. The plate circuit of the thyratron includes the winding II! of the tuning fork drive magnet H6 and the condenser 2", the latter being shunted by the high resistance 2I8. The condenser is therefore normally discharged, and is in condition to charge over a circuit including the thyratron 204 and the winding III whenever the tube has its grid bias lowered enough toenable it to pass current.

- the voltage at the screen grid will be somewhat The voltage regulator VR includes a Type 6080 double triode with the circuit connections as shown. The left-hand triode functions as an amplifier, while the right-hand triode functions as a voltage regulator. The crystal unit l2l of the pickup device at the tuning fork is resistance coupled to the grid of the left-hand triode. One terminal of the crystal unit is connected to ground, as shown, while the other terminal is connected to ground through the two resistors 2|! and 220 in series. These resistors may have a value of 150,000 ohms each. The Junction of the two resistances is connected to the grid oi the left-hand triode.

The plate 01 the left-hand triode is connected to the plus B lead through a-resistor 22l, and the plate circuit is coupled to the grid of the right-hand triode by means 01' a condenser 223 and a resistor 224. The condenser 223 may have a capacity of .25 M. F., while the value of the resistor 224 should be very high, preferably about 5 megohms. The junction point 0 between the condenser 223 and the resistor 224 is connected to ground through the grid leak 225, which may have a value of 500,000 ohms.

The plate circuit 0! the right-hand triode is connected to, the plus B lead througha resistor 225 of about 50,000 ohms and a filter circuit comprising the inductance 221 and the condenser 228. The capacity of the condenser and the value 01' the inductance are such that the filter circuit is sharply turned to 60 cycles. The filter circuit is coupled by means of a condenser 225 to the output lead 252 leading to the power amplifier. The power amplifier comprises an ordinary triode 240 and two Type 6N7 double triodes 2 and 242. The output lead 262 coming from the voltage regulator VB. is coupled to the grid of the triode 240 by means of a potentiometer 235, which constitutes an adjustable grid leak and is used as a voltage control. The plate circuit of the triode 240 is coupled push-pull to the grids of the double triodes 24l and 242 by means of the transformer 243. It will be observed that the two grids of tube 2" are connected in parallel and that the two plates are also connected in parallel, the same being true of tube 242. The plate circuits of tubes 2 and 242 are coupled by means of the transformer 244 to the output leads 253 and 254, which lead to the timer T. At the timer these conductors lead to the winding 5| of the synchronous motor, the circuit including a contact on the switch 9.

The amplifier A2 is similar to the amplifier Al and hence is not shown in detail, being represented merely by a rectangle in which the outgoing and incoming conductors are terminated. The amplifier A2 may in fact be exactly like the amplifier Al except that the condenser corresponding to the condenser 2H and the resistor corresponding to resistor 218 do not have the same values. This will be explained later on in describing the operation of the equipment. The two output leads 261 and 255, which correspond to the output leads 265 and 255, respectively, of the amplifier Al, extend to the timer T and through the slip rings 31 and 38 to the winding 41 of the electromagnet 46. One side of the circuit includes a contact of the switch 9.

' As shown in Fig. 6, the equipment is arranged to beoperated from some alternating current commercial power source, and to this end a plug P may be provided which can be inserted in any convenient socket on the premises. The reference character S represents a main switch by means of which the power circuit is extended from the plug through to the primary winding 25! of the transformer 245.

The transformer 245 is included in the rectifier R and has a number of windings, the function of which will be mentioned briefly. The winding 250 supplies current to the heaters of the various tubes shown throughout the drawings, including those in the two amplifiers AI and A2 and in the voltage regulator and the power amplifier. The heater circuits have not been drawn in, but it will be understood that each heater circuit which is labeled X--X is in practice connected to the terminals X-X of the winding 250. The main rectifier tube 253 is connected for full wave rectification and has the function of supplying plus B potentials to the various tube circuits. The two plates of the tube are connected to the opposite terminals oi? thewinding 245. The midpoint of this winding is grounded and constitues the negative terminal of the rectifier. The cathode of the tube is supplied with heating current from the winding 249 of the transformer and is connected through a suitable filter to the various plus B leads, as illustrated. The highest voltage is supplied to the lead 259 over which power is supplied to the plates of the tubes 24! and 242 of the power amplifier. A somewhat lower voltage is supplied over the conductor 254 to the plate 0! the amplifying triode 240, and a still lower voltage is supplied over leads 255 and 256 to the two amplifiers Al and A2 and to the plates of the tube 205 in the voltage regulator VR.

The rectifier tube 252 is also connected for full wave rectification and is provided for the purpose ofsupplying negative grid bias potential to the thyratrons in the amplifiers Al and A2. The cathode of the tube is heated by means of the winding 24! of the transformer 245 and isconnected to ground. The two plates of the tube are connected to the opposite terminals, re-

spectively, of the winding 246 of the trans- I former 245. The midpoint of this winding constitutes the negative terminal ofthe rectifier and I is connected through a suitable filter circuit to the conductors 258 and 251. The conductor 25! is connected through the resistor 2| 5 to the grid of the thyratron 204 in the amplifier Al, and the conductor 258 is similarly connected in the amplifier A2.

The apparatus having been described, its operation in timing watches will now be explained. It may be assumed that a master watch which has been previously regulated to keep correct time is clamped to the microphone 200. The master watch should. be a five-beat watch and should be adjusted so that the successive beats are evenly spaced. A high grade watch is obviously desirable, and the more perfect its regulation the better, but it need not be regulated for various positions.

The first step on starting up the apparatus is to insert the plug P, Fig. 6, in a convenient socket microphone 200. at each beat or tick of the watch.

a mechanical shock is transmitted to the crystal element in the microphone, in response to which pulse, which The impulse is amplified at the pentode 202 and is passed on through the condenser 2 to the grid of the left triode of the double triode 203, where it is further amplified. The plate circuit of the left triode being coupled to the grid of the right triode through the condenser 2B, the impulse is amplified once more, and is then transmitted through thecondenser M5 to the grid of the thyratron 204, where it reduces the normal negative grid bias enough so that the tube can fire. The action is not as simple as this, in reality. but it is well understood and hence it will not be necessary to go into details.

When the thyratron fires. or starts to pass current, the condenser 2I'I charges over a circuit which may be traced from ground by way of the cathode and plate of thyratron 200, winding H1, and condenser 2|! to the plus B lead 255. The condenser 2|! charges rapidly over the above circuit and as a consequence the current flow through the tube 204 falls rapidly toward a value determined by the high resistance 2 l8, which extingulshes the tube. The condenser then discharges through the resistance 2H8 in preparation for charging again the next time the thyratron fires.

The charging current of the condenser flows through the winding H! of the tuning fork drive magnet H6, as per thecircuit above traced, and accordingly the magnet is energized, attracting the tines I04 and I05 of the tuning fork and pulling them apart. The energization of the magnet H6 is of very short duration. When the tines are released they start vibrating at the natural fork frequency, or 60 cycles per second. The next time the condenser 2H charges, the magnet H6 is energized again, giving another pull to the fork tines. This action is repeated responsive to each beat of the watch, or five times per second, and the fork is very shortly brought up to full amplitude of vibration.

It is essential that the duration of the impulses delivered to the magnet H0 by the charging of the condenser be properly related to the fork frequency in order to secure a satisfactory drive. The'magnet is effective to drive the fork only when the tinesare moving away from each other, or toward the magnet poles, and as the frequency of the fork is 60 cycles per second, this movement takes place in $5 of a second. It has been found by experiment that excellent results are secured if the impulses delivered to the fork are about of this interval, and the capacity of condenser 2 is therefore so related to the impedance of winding III that the condenser will charge up enough to extinguish the thyratron in about of a second.

The winding H1 and the condenser 2" are I therefore so designed as to give the proper length of drive impulses, and this may be done before taking into consideration the proper, response of the thyratron to the watch beats. This latter is taken care of by properly regulating the value of the resistance 2I8. This resistance should be low enough so that it will substantially discharge the condenser 2I'l between watch beats, or in 5 5 of a second, and it should -behighenough so that the condenser cannotdischarge fast enough to permit the thyratron to fire a second time responsive to the same watch beat.

It will be seen now that at each beat of the master watch an impulse is produced which is amplified by the tubes 202 and 203 and which is effective to trigger the thyratron and permit the condenser 2|! tov charge through the tube and the winding Ill of the drive magnet. The impulses thus transmitted to the drive magnet, be ing derived from the master watch, have a frequency of exactly five per second, or A of the fork frequency, and therefore are in step with the fork and serve to maintain it in operation. This is not to say that the fork must have a frequency of exactly 60 cycles per second, although A variationof as much as three or four minutes per day in fork frequency is permissible. When the fork is slightly off, either fast or slow, a correction takes place five times per second, or at each drive impulse.

The exact manner in which the correction takes place is difficult to determine, although careful observations have been made. If the fork has exactly the proper frequency, the drive impulse is apparently delivered as the tines are nearly separated the maximum extent and is terminated slightly after the tines come to rest before starting to approach each other. Now, if the fork is too fast it will gain on the drive impulses, and the tines will start to approach each other on each drive impulse enough before the impulse is completed so that it can exercise a slight retarding effect. On the other hand, if the fork is too slow, the drive impulses will gain on the fork, and each drive impulse will come in at a time when it can deliver more power to the fork than it could if the fork had had exactly the right frequency, with the result that the fork is accelerated. This explanation of course implies that the drive impulses have considerably more power than is necessary to drive the fork when exactly in tune, which is the case. It may be mentioned also that if the fork is considerably off as to its frequency, although not enough to prevent it from running in step with the master watch, its amplitude will be reduced. This, however, is prevented from having any deleterious effect by the voltage regulator, as will be presin a coupling circuit including resistances U0 and 220, whereby alternating potentials are applied to the grid of the left triode of the double triode tube 205. As a result, the plate current fluctuates and the voltage on the plate of the left triode rises and falls. This causes the condenser 223 to charge and discharge in series with the resistor 225, and alternating voltages are produced at the point 0 in the condenser circuit. These potentials vary in amplitude, depending on the amplitude of the voltages applied to the grid of the left-hand triode, which in turn depend on the tuning fork amplitude, and it is the function of the right-hand triode to limit the efiect of both the positive and the negative half waves and maintain a uniform output within certain limits independent of the strength of the impressed voltages.

The effect of the negative potentials is limited by proper adjustment of the grid bias of the right-hand triode. This grid bias is adjusted so .explained up to this point.

that with a negative half wave of the minimum strength some current will still flow through the tube; but if the signal strength is increased somewhat, producing voltage changes at the point of-higher amplitude, the negative half waves will bias the tube to cut oil, and no current will flow during the peak values of such negative half waves.

The positive half waves are limited as to their eflect by means of the resistor 224, which i in series with the grid. This resistance has no effeet during the negative half waves, as the grid is drawing no current, but when the grid swings positive it draws current through the resistor 224. The resistor 224 is very high, being about megohms, and consequently the drop is so high that the potential on the grid can rise only an infinitesimal amount. The result is that no matter what amplitude the positive half wave voltages at the point 0 have, the grid can never become more than a fraction of a volt positive, and the current through the tube is definitely limited to what can fiow with this grid potential.

It will be seen therefore that the right trlode of tube 205 operates as a voltage regulator and supplies a 60-cycle output of substantially uniform voltage independent of the amplitude of the tuning fork. When the fork has the minimum amplitude for which the circuit is adjusted, the output of the right triode of tube 266 will be approximately a sine wave, but with increasing amplitudes the voltage-regulating characteristics come into play and the output has a flattened wave formation. In order to correct this, the plate circuit includes a resonant circuit.comprising the condenser 228 and inductance 221 which is tuned to 60 cycles per second. This tuned circuit is coupled to the output lead 262 through a condenser 229, and the voltages on this output lead are of excellent wave form, approximating a sine wave, and are of substantially uniform voltage, irrespective of varying fork amplitudes,

within wide limits.

The power amplifier comprising the triode 240 and the power tubes 24| and 242 functions in known manner, receiving 60-cycle alternating potentials from the output lead 262 of the constant frequency generator and delivering 60-cycle al ternating current to the output leads 262 and 264.

There are two controls which are used in ad-' justing the equipment whose operation has been The potentiometer 2|2 is used as a volume control to adjust the amplifier 4| in order to secure a regular firing of the thyratron 204 responsive to the beats of the master watch. The potentiometer 236 is used to adjust the output voltage of the power amplifier to the desired value. When once adjusted these controls need no attention for long intervals.

The switch 9 may now be closed, thereby bridging the field winding 3| of the synchronous motor in the timer T across the output leads 263 and 264. The motor may be self starting, or it H may be of the type shown in my parent application, in which case it is started by means of a special manually operated device which gives the armature an initial spin at a high enough speed to enable it to fall into step with the current supply.

The synchronous motor in the timer T i now being driven by the master watch through the intervening apparatus including the beat amplifier Al, the tuning fork F, the voltage regulator VR, and the power amplifier PA. This apparatus serves to convert the five-cycle beat frequency output of the microphone 260 into an amplified 60-cycle current having power enough to run the motor. The motor runs at five revolutions per second, and to this end the armature 24 is provided with twenty-four teeth. By means of shaft 20 the motor rotates the magnet 46 in a counter clockwise direction, as viewed from the left in Fig. 6. The dial 62 is stationary.

A watch to be tested may now be clamped to the microphone 260. It may be assumed that the watch is a five beat watch. The microphone responds to the watch beats and generates impulses which are amplified by the amplifier A2 and are effective to control the thyratron in this amplifier in the same manner as described in the case of thyratron 204 in amplifier AI. The plate circuit of the thyratron includes the two output leads 261 and 266, corresponding to leads 265 and 266 of amplifier Al; and when the thyratron fires the condenser which corresponds to condenser 2|1 charges over a circuit which includes the cathode and plate of the thyratron, lead 261, winding 41 of magnet 46, switch 9 (now closed), lead 266, and the condenser. As soon as the condenser becomes substantially charged, current flow is reduced sufliciently to extinguish the thyratron, and the condenser then discharges through a resistance corresponding to resistance 2|8. The action is the same as described in the case of thyratron 264 and condenser 2|1, except that the capacity, impedance, and resistance values are so chosen that the impulses delivered to the magnet 46 are much shorter than those delivered to the drive magnet N6 of the tuning fork. Impulses not over 75 00 of a second in dura-. tion give good results.

It should be recalled now that the magnet 46 is rotating in a counterclockwise direction at a speed of five revolutions per second. The first impulse received will come in at a time when the magnet 46 is in some random position relative to the armature 5|. If the magnet and armature are in such a relative position that the magnet poles are aligned with a pair of armature poles (the armature may have three pairs of opposite poles no rotation of the armature will ensue; but if the magnet is in such a position that the magnet poles are not in alignment with a pair of armature poles, the nearest pair of armature poles will be attracted and the armature will be rotated. This rotation is opposed by the resistance developed at a suitable friction device, and by the inertia of the armature 5| and dial 62, and the adjustment is such that, having regard to the very short duration of the impulses, the maximum amount of rotation that can take place responsive to a single impulse is about four or five degrees. Several impulses may be required therefore in order to position the armature 6| with relation to the effective position of the rotating magnet 46. When this has been accomplished no further rotation of the armature 6| will take place, if the watch being tested is keeping correct time with respect to the master watch, as the magnet 46 will be in the same position relative to the armature each time the magnet is energized. This will be clear from the fact that the magnet 46 rotates five times per second and from the further fact that the watch being tested is a five beat watch.

From the foregoing it will be evident that the operator viewing the dial 62 will usually observe a movement of the pointer associated with the scale in one direction or the other immediately upon placing the test watch on microphone I".

This movement is of course recognized as being due to the initial positioning of-the armature relative to the rotating magnet and is disregarded. If the test watch is keeping correct time,'no further movement of the pointer will occur, and the operator knows from this that the watch is correct. 1

Assuming now that the test watch is running too slowly, the rotating magnet will gain on the impulses which it receives from the test watch and will rotate slightly more, than a complete revolution between successive impulses. As each impulse is received, therefore, the magnet will be in a slightly advanced position where it can exert apull on the armature,.tending to rotate the dial and pointer in a counter clockwise direction. The pointer will therefore move gradually to the left, indicating to the operator that the test watch is running slow.

Assuming, on the other hand, that the test watch runs too fast, the impulses delivered to the rotating magnet 46 will gain on the magnet, which will make somewhat less than a complete rotation between successive impulses. As each impulse is received, therefore, the magnet will be in a retarded position relative to the armature and will rotate it in a clockwise direction, moving the pointer to the right, which indicates to the operator that the test watch is'too fast.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be pointed out in the appended claims.

I claim as my invention: v

l. The combination, with means for generating alternating potentials .varying in voltage, of voltage-regulating means comprising a vacuum tube, a circuit for applying said potentials to the grid of said tube, means for applying a negative bias to said grid to limit the effect of the negative half waves, a high resistance in series with said grid to limit the effect of the positive half waves, and a filter in the plate circuit tuned to 5 the frequency of said alternating potentials to eliminate the defects in wave form introduced by said limiting means.

2. In combination, a 'ondenser, a resistance,

a circuit for-dischargingjsai'd condenser through said resistance, a gaseous spa'ce current device connected in series with said condenser and a source of current, means for periodically rendering said device conductive to charge said condenser, a tuning fork,.and aidriving magnet for 5 said fork energized by the charging currents.

3. In combination, means responsive to the beats of a watch for generating impulses of low frequency, means for converting said impulses into alternating current of a higher frequency, means for correcting the wave form of said current, means for amplifying said current, and a synchronous motor operating on said amplified current.

4. In combination, a source of impulses of substantially constant low frequency, a tuning fork which has a natural period which is substantially-a multiple of the said impulse frequency,

means for causing said impulses to drive saidfork at. an amplitude which varies with the dif- 10 ference between the impulse frequency and the 7 fork frequency, means responsive to operation of said fork for generating alternating current at a potential which varies with the amplitude of 2,285,317} i said fork, means for automatically limiting the which bears a constant relation to the frequency of said source, means for altering the wave form of the generator output to a substantially sine wave form, and means for amplifying the generator output to produce alternating current for driving such a synchronous motor.

6. In combination, a source of periodic currents of extremely constant frequency which is however unsuitable for amplification for operating a synchronous motor, an oscillatory generator having a suitable natural frequency of inferior constancy, means for utilizing the output of said source to drive said generator at a fre-. quency which approximates its natural frequency and which bears a constant relation to the frequency of said source, means for limiting the voltage of the generator output to a predetermined value, means for altering the wave form of the generator output to a substantially sine wave form, and means for amplifying the generator output to produce alternating current for driving such a synchronous motor.

7. In a constant frequency generator, a tuning fork, a magnet having its opposite poles located outside of and near the ends of the two tines of said fork, respectively, means for periodically energizing said magnet to drive said fork, a piezoelectric crystal generator, means mechanically coupling said crystal generator to one tine of said fork near the Junction of said tine with the other tine and remote from said magnet, and means for amplifying the output of said crystal gen erator. V

8. In combination, a source of periodic currents" of extremely constant frequency which is however unsuitable for amplification for operating amotor, an oscillatory generator adapted to operate at a frequency suitable for amplification but having inferior constancy, means including a space discharge device for utilizing the output of said source to drive said generator at a frequency which at least approximates said suitable amplification frequency and which bears a constant relation to the frequency of said source, means for altering the wave form of the generator output to a substantially sine wave form, and means for amplifying the altered generator output to produce alternating current for driving the mofork near the junction of said tine with the other tine and remote from said magnet, and means for amplifying the output of said crystal gen- 

